1. Technical Field of the Invention
This invention relates to critical temperature indicators. More particularly, it relates to critical temperature indicators which provide an irreversible visual signal to the user that a product has been exposed to a predetermined temperature, usually near the freezing point of water.
2. Description of the Prior Art
Blood, emulsions, pharmaceuticals, beverages, and other items are often chilled in order that they may be preserved for use sometime later. However, the characteristics of these items may change so that they are undesirable for later use or the items may be rendered completely useless if exposed to a lower temperature, usually the freezing point of water. It is, therefore, desirable to provide an indicator device which will accurately show whether or not the temperature of an item or the environment has decreased below a critical value.
It is also desirable that the device provide its indication rapidly and that the indication be irreversible so that the user will be alerted to a past frozen condition, even if that condition does not presently exist.
Many critical temperature indicators have been provided which utilize the expansion characteristic of water to break a frangible ampule as, for example, Couch et al U.S. Pat. No. 4,145,918. Once the device shown in the Couch et al patent is exposed to temperatures below the freezing point of water, the volume increase as the water changes to ice causes the ampule to break. After the ice has formed and after the surrounding temperature has returned to a point above the melting point of the ice, the water is absorbed by a dye loaded pad, thus providing an indication that the device had gone through a freeze stage and back through a thaw stage.
Problems have arisen in giving an accurate indication of the passage of the device through the normal freezing point of water, i.e., 32.degree. F. (0.degree. C.), due to the super-cooling effect of water which will permit water in small containers to stay in its liquid state substantially below its normal freezing point. Under some conditions, water may be cooled to as low as 3.2.degree. F. (-16.degree. C.) without freezing. This problem has been partially overcome by the addition of certain nucleating agents to the water.
U.S. Pat. No. 4,191,125, issued to Johnson, for example, discloses that cupric sulfide powder may be used as a nucleating agent to reduce the tendency of water to super-cool. The use of such a nucleating agent, however, has not proven entirely successful since an example in the Johnson patent indicates that out of several hundred of the devices exposed to an environment at 26.6.degree. F. (-3.degree. C.), no devices were activated after one hour.
Such an inaccurate performance is undesirable and it is an object of this invention to provide an indicator which will avoid such inaccuracy.